1. Field of the Invention
The present invention concerns a method and a device in order to generate images of a volume segment using a magnetic resonance system.
2. Description of the Prior Art
At present, in cardio-MRT measurements for cine-imaging different slices are typically measured at different positions in succession, for example in order to arrive at a volumetric image coverage to determine the cardiac function. Cardio-MRT measurement thereby stands for cardiovascular measurements by means of a magnetic resonance tomograph (MRT). Cine-imaging as used herein means a series or multiple series of images that are generated quickly at different points in time and are shown as a type of movie. In other words: MR measurements of the heart are produced by means of cardio-MRT measurements in order to show a moving heart.
Three-dimensional methods exist for this purpose, but the image quality is, however, often not comparable with the image quality of two-dimensional methods due to the slice profiles and additional aliasing in the direction perpendicular to the slice. Moreover, the MR signal to be acquired is strongly saturated by the lingering volume excitation (in comparison to two-dimensional methods), which is reflected in a poor image contrast.
In the two-dimensional methods, a heart beat is typically brought into a dynamic equilibrium (steady state) relative to the magnetization along the magnetization within the scope of a type of initialization without MR signals being thereby measured or image data acquired, such that a change of the MR signal across k-space measured as a whole for an image no longer exhibits any significant signal fluctuations resulting from this in the actual imaging or MR measurement. If this initialization to engage the magnetization is omitted, for example in order to save the time of the heart beat, the first images exhibit corresponding artifacts, which negatively affects the quality of the imaging.
In strongly segmented measurements in which many measurements (for example 16) per slice are implemented, an additional heart beat for the initialization means no noteworthy time loss since a length of many heart beats (at least if only one measurement is conducted per heart beat) must be measured anyway for a complete measurement of a slice. Based on the developments of the last year, most of all in the field of parallel imaging, however, it is presently possible to collect all data or, respectively, MR signals necessary for a slice with a few segments, or even with only one segment (a single shot), with sufficient spatial and temporal resolution. Therefore, in the extreme case the additional heart beat leads to a reduction of the process efficiency of 50% in a real time imaging.
According to the prior art, it is accepted that an additional heart beat is necessary to allow the magnetization to take effect. Therefore, according to the prior art of the technique the minimum measurement duration of a slice is two heart beats if it is assumed that only one measurement can be implemented per heart beat. In a typical breath hold phase of 12 s, a maximum of six slices can thus be measured. For a measurement to evaluate the heart function, at least two breath hold phases are therefore necessary according to the prior art.